The "hate" for him comes from how much he homogenized Trek, distilled it down to pretty much just a product. He watered down the music, he preferred episodic story-telling and pretty much ran the series (aside from DS9) on minimal life-support, just enough to keep money flowing in but not enough to allow them to be truly fantastic or dramatic. He was VERY hesitant about taking on risky subject matter or doing anything bold. He wanted it as a product to put money in his pockets, not use it for artistic enlightenment or to do bold story telling.

The studio deserves at least as much blame for this, probably most, rather than Berman personally. Their executives have a great deal more power than most fans realise.

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"STAR TREK is... Action - Adventure - Science Fiction."
-- Gene Roddenberry, 1964, top of the first page of his original pitch and outline for Star Trek